Methanol is a commodity used in various areas. It can be converted to formaldehyde which is used to make synthetic polymers or to mix with other materials. Methanol can also be used to make a wide variety of chemicals including dimethyltoluene, methylamines, chlorinated solvents, acetic acid, isoprene, or used in fuels. Elevated environmental standards and limited oil supplies can yield an increasing demand in methanol in the future.
The synthesis of methanol can be achieved through a synthesis gas (syngas). Syngas contains hydrogen (H2) and carbon monoxide (CO), may further contain other gas components, e.g., carbon dioxide (CO2), water (H2O), methane (CH4), and/or nitrogen (N2). Carbon oxides react with hydrogen to form methanol according to the following equations:CO+2H2=CH3OHCO2+3H2=CH3OH+H2O
The composition of syngas is characterized mainly by its H2 and CO content; generally presented by the so-called stoichiometric number (SN), which is defined as:SN=([H2]−[CO2])/([CO]+[CO2])where the concentrations of components are expressed in vol % or mol %.
In the past, processes have been developed to produce syngas. Natural gas and light hydrocarbons are the predominant starting materials for making syngas. Such syngas processes frequently use methane as a starting material, which may be converted to syngas by steam reforming, partial oxidation, CO2 reforming, or by a so-called auto-thermal reforming reaction. One technology for producing syngas from a methane feedstock is the reaction with water (steam) at high temperatures, generally called hydrocarbon steam reforming.
If a feedstock is used in a reforming process that is rich in higher hydrocarbons, like naphtha, the feedstock first needs to be treated in a so-called pre-reforming step, in order to convert the heavy hydrocarbons in the feed into methane, hydrogen and carbon oxides. Such higher hydrocarbons are more reactive than methane in steam reforming, and would—if present in the feed—lead to carbon formation and thus to deactivation of the catalyst employed in steam reforming. In such a pre-reformer, several reactions take place simultaneously; the most important being hydrocarbon steam reforming (1), water gas shift (2), and methanation (3) reactions, which can be represented, respectively, as:CnHm+nH2O⇄nCO+(m/2+n)H2  (1)CO+H2O⇄CO2+H2   (2)CO+3H2⇄CH4+H2OCO2+4H2⇄CH4+2H2O   (3)Such a pre-reformer is typically operated adiabatically at temperatures between about 320° C. and 550° C.
In a steam methane reformer (SMR), methane-rich gas is converted into a mixture containing carbon monoxide, carbon dioxide, hydrogen and unreacted methane and water in the so-called steam reforming (4) and carbon dioxide reforming (5) reactions, represented as:CH4+H2O⇄CO+3H2   (4)CH4+CO2⇄2CO+2H2   (5)
One disadvantage associated with syngas production by methane steam reforming is that the composition of the produced gas mixture is limited by the reaction stoichiometry to H2/CO ratios of 3 or higher.
In order to avoid such drawbacks and to help counteract increasing CO2 concentrations in the atmosphere, attempts have been made to manufacture syngas from CO2 as a raw material. The conversion is based on the following equation:CO+H2O⇄CO2+H2 
The forward reaction is known as the water gas shift (WGS) reaction, while the reverse reaction is known as the reverse water gas shift (RWGS) reaction.
Conversion of CO2 to CO by a catalytic RWGS reaction can be a process for CO2 utilization. Early work proposed iron oxide/chromium oxide (chromite) catalysts for this endothermic reaction; see, e.g., U.S. Pat. No. 1,913,364. However, such catalysts can suffer from methane formation and the accompanying catalyst coking problem.
GB 2168718A discloses combining the RWGS reaction with steam reforming of methane. The combination of the two reactions allowed the molar ratio of H2:CO to be adjusted and to better control the SN value in the final syngas mixture to values of about 3 or higher, depending on the intended subsequent use of the syngas mixture.
GB 2279583A discloses a catalyst for the reduction of CO2, which comprises at least one transition metal selected from Group VIII metals and Group VIa metals supported on ZnO alone, or on a composite support material containing ZnO. In order to suppress CH4 formation and catalyst deactivation, stoichiometric H2/CO2 mixtures and low reaction temperatures were used, which resulted in relatively low CO2 conversion.
U.S. Pat. No. 5,346,679 discloses the reduction of CO2 into CO with H2 using a catalyst based on tungsten sulphide. U.S. Pat. No. 3,479,149 discloses using crystalline aluminosilicates as catalyst in the conversion of CO and water to CO2 and H2, and vice versa.
U.S. Pat. No. 5,496,530 discloses CO2 hydrogenation to syngas in the presence of nickel and iron oxide and copper or zinc containing catalysts. In WO 96/06064A1, a process for methanol production is described, which includes a step of converting part of the CO2 contained in a feed mixture with H2 to CO, in the presence of a WGS catalyst exemplified by Zn—Cr/alumina and MoO3/alumina catalysts.
WO 2005/026093A1 discloses a process for producing dimethylether (DME), which includes a step of reacting CO2 with H2 in a RWGS reactor to provide carbon monoxide, in the presence of a ZnO supported catalyst; a MnOx (=1˜2) supported catalyst; an alkaline earth metal oxide supported catalyst and a NiO supported catalyst. EP 1445232A2 discloses a RWGS reaction for production of CO by hydrogenation of CO2 at high temperatures, in the presence of a Mn—Zr oxide catalyst.
United States Patent Publication No. 2003/0113244A1 discloses a process for the production of a syngas mixture that is rich in CO, by converting a gas phase mixture of CO2 and H2 in the presence of a catalyst based on zinc oxide and chromium oxide, but not including iron. The presence of both Zn and Cr was indicated to be essential for formation of CO and H2 mixture at a good reaction rate, whereas the presence of Fe and/or Ni is to be avoided to suppress formation of CH4 via so-called methanation side-reactions. Formation of CH4 as a by-product is generally not desired, because its production reduces CO production. The co-production of CH4 may also reduce catalyst life-time by coke formation and deposition thereof. A drawback of the process for syngas production disclosed in U.S. 2003/0113244A1 is the selectivity of the catalyst employed; that is CH4 formation from CO2 is still observed as a side-reaction. In the illustrative example, this CH4 formation was quantified as 0.8 vol % of CH4 being formed in the gas output of the reactor, at a degree of conversion of CO2 of 40%.
In addition, U.S. Patent Publication Nos.: 2010/0190874 and 2010/0150466 disclose processes of making syngas including CO, CO2 and H2 under an isothermal conditions by contacting a gaseous feed mixture including CO2 and H2 with a catalyst including Mn oxide and an auxiliary metals, e.g., La, W, etc.
There remains a need in the art for improved and less energy consuming processes for synthesizing methanol.